This invention relates to flat cable and in particular flat cable comprising a plurality of conductors and a substrate comprising a polymeric material.
Flat cables are well known and take several forms. In one form of flat cable a plurality of conductors are placed parallel to one another and laminated between a pair of films which surround each conductor thereby insulating each conductor from its neighbors as well as forming the flat cable construction. The insulation thickness of each individual conductor cannot always be controlled as precisely and reproducibly as desired.
Another form of flat cable, in which pre-insulated conductors are bonded to a carrier film, addresses this problem. The film is generally of a polymeric material similar to that of the insulation and in order to achieve a bond between the insulated conductors and the film, the film must be heated to about its melting point. At these temperatures it is difficult to maintain the dimensions of the film and this can lead to improper alignment of the wires and even buckling and shrinking of the film. Further, it has been found that with certain polymeric materials it is difficult to separate an individual insulated conductor from the carrier film unless the bond strength is well controlled. If the bond is too strong, peeling will then destroy the carrier film or the conductor insulation. It is desirable to be able to peel the individual insulated conductors from the film as cleanly as possible. All such problems, we have discovered, are prevalent, and sometimes unavoidable using prior art methods, when polymer film is heated above its melting point to enhance adhesion, especially when polymer film has a high melting point, for example, over 200.degree. C. or when the film is thin, for example less than 2.0 mil. We have also discovered that, for optimum adhesion, the film should be heated to a higher temperature close to the ring and ball softening temperature of the polymer material. We have also found that when using prior art methods of thermal bonding at a sufficiently high temperature to ensure good adhesion, the bond strength between the cable insulation and the carrier film varies erratically and even under nominally identical process conditions, the average bond strength can vary widely from one production run to the next.
U.S. Pat. Nos. 4,625,074 and 4,678,864 to Cox, and 4,767,891 to Biegon et al. describe attempts to obtain a flat cable having dimensional stability at elevated temperatures during the manufacture of the cable. These patents disclose the use of a carrier film comprising a laminate of an attachment layer of a first polymeric material and a dimensional stabilization layer of a material having a higher melting point than the first polymeric material. The attachment layer and the dimensional stabilization layer are joined together with an adhesive layer. In use, the dimensional stabilization layer remains part of the flat cable, the attachment layer serving merely as a bonding layer and the flat cable can therefore delaminate if the cable is subject to temperatures above the melting point of the adhesive or attachment layer.
U.S. Pat. No. 4,678,864 to Cox also discloses the use of a ground plane in the construction of a flat cable assembly. The ground plane is positioned between the flat cable and a plastic covering. The plastic covering is coated over a major portion of its surface with a release agent so that it can be readily separated from the cable. The ground plane itself is not bonded to the flat cable. The flat cable itself is of conventional structure comprising a supporting film and insulated wires and thus it would be difficult to peel individual insulated wires cleanly from the cable.
This application is related to copending, commonly assigned, U.S. application No. MP1441-US1, Ser. No. 07/847,558, filed Mar. 6, 1992.